The present invention relates to a probe for temperature measurement, comprising a probe body which has a cylindrical hole into which a thermocouple can be inserted, whose thermosensitive tip projects from the forward section of the probe body.
Such probe bodies are known. As a rule they are cylindrical in shape and are conically tapered at the forward end. At the rear end they are provided with a thread by means of which they can be screwed into a mounting. Into the cylindrical hole a thermocouple of known design, preferably a miniature sheathed thermocouple is inserted. Such thermocouples normally comprise two cores of different metals which are embedded into an insulating material and are surrounded by a metallic shield, the sheath, the insulating material used being magnesium oxide or aluminum oxide, which have a high dielectric resistance. The sheath materials used as a rule are stainless steels. The thermocouple cores used are customarily NiCrxe2x80x94Ni wires which are welded together at the forward end. Above 1200xc2x0 C., platinum-rhodium-platinum thermocouples are also used.
Thermoprobes containing thermocouples of this type are used for many purposes. Problems arise if they are used for temperature measurements in reactors and reactor tubes which are under high pressure and which require rapid temperature acquisition. The high pressure entails thick walls for the probe body, which must be of robust design. This results in a relatively sluggish change in temperature, which is transmitted to the tip of the thermocouple. Plants where such thermoprobes are used include, for example, high-pressure polyethylene plants in which the reaction is carried out by ethylene compression at operating pressures between 1500 and 3500 bar. In such a reactor, approximately 40 thermo-probes are employed whose purpose is to map the corresponding temperature profile, the probe bodies as a rule having a diameter of from 8 to 10 mm. The diameter of the thermocouples inserted into these is customarily approximately 2 mm.
U.S. Pat. No. 4,749,415 is discloses a thermocouple element for use in high-pressure, high temperature fluid applications having a cylindrical body portion formed from the first metal and a cap formed from a dissimilar metal. The cap is fused to the body such that the junction forms an axial thermocouple function. A plurality of grooves is formed in the cap and body to reduce the mass of material adjacent the unction for rapid conduction heat transfer from the fluid being monitored. The bottoms and the openings of the grooves are arranged parallel to the axis in lengthwise direction, such that the grooves extend in a radial direction towards the axis. This results in an increased surface submitted to the flow and therefor the conduction heat transfer is increased.
It is an object of the present invention to provide thermoprobes which can be used in reactors involving very high pressure and very high temperature, and a high flow velocity of the reactants and which nevertheless are able to detect rapid changes in temperature.
We have found that this object is achieved, according to the invention, by virtue of the forward section of the probe body being provided with an annular gap at a small distance from the cylindrical hole. Said annular gap has as low a thickness as possible, as a rule from 0.1 to 0.5 mm, preferably from 0.1 to 0.2 mm. The length of the annular gap is from 5 to 20 mm, preferably from 10 to 15 mm. The annular gap may have been produced by means of a laser beam. It may also be produced by virtue of the forward section of the probe body having at least one stepped reduction in the diameter, onto which reduction a cap has been fitted whose internal diameter in the forward section is greater than the forward section of the probe body, the annular gap being formed as a result. The cap may be welded to the probe body.
The probe according to the invention is particularly suitable for rapid temperature acquisition in plant sections where very high pressures, temperatures and flow velocities will occur. This is the case in chemical reactors, stirred vessels, tubular reactors, feeders to these apparatuses, and piping connections. Preferably, such probes can be used in polymerization reactors, both in the gas phase and in the supercritical medium, for example in the preparation of polyolefins. In the case of suspension polymerization in loop reactors these probes can likewise be employed advantageously.
In general this involves pressure ranges of from 30 to 5000 bar, preferably from 50 to 3500 bar, in particular from 1000 to 3500 bar, and temperatures of from 20 to 1000xc2x0 C., preferably from 50 to 500xc2x0 C., in particular from 100 to 400xc2x0 C. The flow velocities are in the range from 1 to 50 m/s, preferably from 5 to 15 m/s.
The thermocouples used can be miniature sheathed thermocouples of various designs, preferably those made from NiCrxe2x80x94Ni wires, which are welded together at the forward end. Platinum-rhodium-platinum thermocouples and other known thermocouples can likewise be employed in this instance. Said wires of the thermocouple are embedded in an electrically nonconductive insulating material such as magnesium oxide or aluminum oxide and are surrounded by a metallic shield, the sheath. At the forward end the thermocouples are joined to the sheath by a metallic cap. Said cap must be insensitive with respect to the high temperatures, pressures and flow velocities arising. The cap is preferably hemispherical in shape.
As an alternative to the thermocouples, it is also possible to employ resistance thermometers or measuring shunts, platinum resistance thermometers and platinum measuring shunts being particularly advantageous, for example according to DIN IEC 751. These resistance thermometers comprise a measuring shunt, responsive to temperatures, in a protective cover, internal supply leads and external terminals for connection to electrical measuring instruments.